Saturday, August 04, 2007

I post about turbidites a lot on this blog. I have never included a link to an online explanation of them because I am simply not satisfied with them. I should create a post of my own to do this but....well, that's a lot of work and I don't want to do it half-ass....you all know how it is.If you have no idea what a turbidite is....then check out the Wikipedia turbidite page. It's only okay...some of it is not quite right and there's a severe lack of external links.

Like many geoscientists, I find that visuals help immensely in understanding processes. We can go to a modern river, delta, tidal inlet, etc. and observe firsthand the processes. We cannot do this in the deep water; our monitoring of processes in this environment is extremely limited and still more-or-less indirect.

This is where lab experiments have value. But, turbidity currents that have been observed in historical times can be huge. The earthquake-triggered turbidity current that occurred offshore of eastern Canada in 1929 is estimated to have been 400 m tall, lasted for at least 12 hours, and traveled hundreds of km into the abyssal plain (note: I will post about that story another time...it is really cool). So, we have to scale them down. There are problems with assuming scale-independence when it comes to sedimentary processes, but, as a general exercise, we have learned a great deal from experiments.

This video below (via Paul Heller's website) shows an experimental turbidity current in a tank. It is kind of long (>3 min)....you'll get the idea after 30 seconds or so. Remember, this is subaqueous...notice the "head" of the current plowing its way through the water. The ambient fluid does not get incorporated into the current at the head, but rather is displaced over the top and mixes with the more dilute cloud. In this way, the turbidity current grows over time. The head also travels faster than parts of the flow behind it, which results in a lengthening of the current over time and distance. If you do watch the video to the end, you'll get an appreciation of how real turbidity currents can last so long....it simply takes a long time for them to come to rest. The more volume, the longer the duration.

Stay tuned for more posts about the fundamental processes of turbidity currents and the deposits they create.